You can purchase pie shaped magnets where the pointy ends are truncated. They are used to make DC motors. You buy one kind so that all the similar poles faces outward, rather than alternating as in a motor. You'll end up with a thick ring with a fairly large hole in the middle. Google "motor magnets". I suggest you only buy them online if the supllier clearly states the orientation of the poles.

You can get arc shaped magnets with the magnetic orientation you're looking for at rare-earth-magnets.com. I hope it's what you need. :) I would have posted a direct link, but I don't have 15 posts yet.